1. Field of the Invention
The present invention relates to a laser annealing method and an apparatus for determining the laser annealing conditions.
2. Description of the Related Art
Presently, liquid crystal displays (LCD) provided with an insulated gate type thin film transistor (TFT) using an amorphous silicon (a-Si) as a pixel switching element are being produced on a mass production basis. However, a TFT using an a-Si having a low electric field mobility (xcexcFE), which is not higher than 1 cm2/Vs, is insufficient for realizing a high performance display of, for example, high precision and high speed.
On the other hand, an electric field mobility xcexcFE of about 100 to 200 cm2/Vs has been obtained in an experimental TFT, by using a polycrystalline silicon prepared by laser annealing, in which an a-Si is irradiated with an excimer laser. This is expected to realize a high performance display of, for example, high precision, high operating speed, and to also integrally contain a driving circuit.
In excimer laser annealing (ELA), an a-Si deposited on a glass substrate is irradiated with an excimer laser so as to convert the a-Si into a polycrystalline silicon. In this method, the excimer laser beam has, for example, a length of 250 mm and a width of 0.4 mm on the surface of the a-Si. The pulse beam is oscillated at 300 Hz, and the region irradiated with each pulse is gradually moved so as to convert the a-Si into a polycrystalline silicon. The liquid crystal displays equipped with a TFT prepared as above are collectively called low temperature polycrystalline silicon LCDs.
The grain size of the polycrystalline silicon, which is the factor for determining the electric field mobility xcexcFE of the polycrystalline silicon TFT, is greatly dependent on the energy density, or fluence of the irradiating laser beam. To be more specific, with an increase in the fluence, the grain size of the polycrystalline silicon is also increased. In order to obtain a high performance polycrystalline silicon having an electric field mobility xcexcFE not lower than 100 cm2/Vs, required is a fluence higher than the value of a certain fluence, F1. Incidentally, xe2x80x9cfluencexe2x80x9d denotes the value obtained by integrating the laser irradiation amount per unit area with time.
If the fluence value is increased to exceed F1, the grain size of the polycrystalline silicon is further increased. However, if the value of the fluence is further increased to exceed F2, the polycrystalline silicon grains are converted into microcrystalline grains. It is impossible to obtain a TFT having the desired characteristics by using such a microcrystalline silicon.
The grain size of polycrystalline silicon can be determined by a so-called xe2x80x9cSecco-etching methodxe2x80x9d, in which the polycrystalline silicon is etched with a Secco-etching solution and the etched surface is observed and measured by a scanning electron microscope. By using this method, a fluence value of a laser corresponding to a region in which the grain size of the polycrystalline silicon is somewhat large is selected. The fluence value thus selected falls between F1 and F2. It is thus possible to obtain a TFT having a desired electric field mobility, even if the laser oscillation intensity greatly varies.
However, Secco-etching requires the steps of cracking the substrate, applying an etching and observing the etched surface with an FE-SEM and, thus, it takes a long time to determine the grain size. Therefore, Secco-etching leads to low productivity.
Also, it is known to the art that the optimum condition of the laser is changed with increase in the number of shots after the gas replacement in the laser device, making it necessary to obtain the optimum condition frequently. Naturally, it is impractical to employ Secco-etching whenever the optimum condition is obtained.
An object of the present invention is to provide a laser annealing method that permits obtaining a TFT having a high mobility over the entire substrate surface with a high yield.
Another object of the present invention is to provide an apparatus for determining the condition of the laser annealing.
According to an aspect of the present invention, there is provided a laser annealing method, in which an amorphous silicon thin film is irradiated with a laser beam so as to convert the amorphous silicon into polycrystalline silicon, comprising: irradiating an amorphous silicon film sample with a laser beam for determining the condition under various energy density conditions so as to prepare a plurality of polycrystalline silicon film samples having different grain sizes; allowing a light beam having a wavelength region including a visible wavelength in a center to be incident on the polycrystalline silicon film at a first angle relative to a line normal to a surface of the polycrystalline silicon film and detecting scattered light or reflected light at a second angle corresponding to a direction different from a direction of a regular reflection of the incident light so as to measure degrees of surface scattering of the plural polycrystalline silicon film samples and, thus, to obtain energy density condition corresponding to the polycrystalline silicon film sample having highest degree of scattering; adding a certain value of energy density to the energy density condition obtained in the preceding step so as to determine a set value of the energy density; and irradiating the amorphous silicon thin film with a laser beam at the set value of the energy density determined in the preceding step so as to carry out the laser annealing.
According to another aspect of the present invention, there is provided an apparatus for determining the condition used in a laser annealing method, in which an amorphous silicon thin film is irradiated with a laser beam so as to convert the amorphous silicon into a polycrystalline silicon, comprising: a light source arranged to permit a light beam having a wavelength region including a visible wavelength in a center to be incident on a plurality of polycrystalline silicon film samples having different grain sizes, the polycrystalline silicon film samples being prepared by irradiating amorphous silicon film samples with a laser beam for determining the condition under various energy density conditions, at a first angle relative to a line normal to a surface of the polycrystalline silicon film sample; a detector arranged to permit the scattered light or reflected light from the polycrystalline silicon film sample to be detected at a second angle corresponding to a direction different from a direction of a regular reflection of the incident light; a mechanism for obtaining energy density condition corresponding to the polycrystalline silicon film sample having highest degree of scattering by measuring degree of surface scattering of the plural polycrystalline silicon film samples by the detector; and a mechanism for determining a set value of the energy density by adding a certain value of the energy density to the energy density condition obtained by the mechanism for obtaining the energy density condition.
Additional objects and advantages of the present invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the present invention. The objects and advantages of the present invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.